1. Field of the Invention
The present invention relates to a disc storage unit for writing or reading information into or from the surface of a disc-shaped recording medium such as a magnetic disc, or erasing information recorded thereon.
2. Description of the Prior Art
Known examples of disc storage units are optical disc storage units, compact disc storage units, video disc storage units, opto-magnetic disc storage units magnetic disc units.
Magnetic disc units, such as floppy disc storage units or hard disc storage units, have been widely with computer systems, word processors image processing units and so on, as external memory systems.
In the case of the magnetic disc unit, a recording medium such as a floppy disc, a hard disc or the like is rotated, while a magnetic head is displaced in the radial direction of a surface of the disc so as to be positioned at a plurality of positions displaced from each other by a predetermined pitch on the surface of the disc to write information. As a result, information tracks with a predetermined width are formed on the surface of the disc, so that the written information can be read out from the information tracks or the written information in a particular track can be erased.
There is the possibility that a track position on the surface of the disc or the position of the magnetic head may vary due to expansion or contraction caused by variations in environmental conditions, due to slight inaccuracies during disc fabrication, and so on, so that a certain amount of off-track operation (that is, a deviation of the head from its correct position) must be taken into consideration.
Therefore, in a disc storage unit in which the position of a track is controlled by an open loop control system, a guard band is defined between adjacent tracks or tunnel erasing is performed in order to minimize crosstalk between the adjacent tracks.
Meanwhile, recently there has been proposed a system for a magnetic disc storage unit for controlling the position of a magnetic head in response to a servo signal or a track servo signal recorded on both sides of each track. The servo signal is written into a surface of a magnetic disc after information recorded on the magnetic disc has been all erased prior to formatting of the magnetic disc. In this case, a formatter; i.e., a disc writer exclusively used for writing a disc has been used to write the servo signal.
In the case of a magnetic disc unit in which each magnetic disc is used in the manner described above, the position of each track can be controlled with a high degree of accuracy and furthermore the guard band can be eliminated, so that the data storage density can be increased.
However, in the case of a magnetic disc storage unit of the type described above, only a recording medium which has been formatted by a formatter exclusively for this type of disc storage unit, to record ID information and a track servo signal, can be used. It is not possible for users to format a raw disc which has not been formatted, as in a case of a conventional open loop control system type unit. In addition, the formatted disc is expensive, so that the operating cost of the recording medium is also increased.
In view of this, one might consider a magnetic disc drive unit that functions per se to write the servo signal without using a special formatter in order to write the servo signal of the type described above. In this case, no problem arises in the writing mode, but the following problem arises in the erasing mode to be accomplished prior to the writing mode.
That is, in the case of a magnetic disc drive unit of the type in which the position of the head is controlled in response to the servo signal, the tunnel erasing system is not employed in order to increase the data storage density and the track width is narrower than the track pitch. As a result, even if recorded information is erased at the normal track pitch, there remains a portion of the magnetic disc in which the recorded information has not been completely erased. Therefore, when a non-erased portion is left, there is the possibility that this portion may degrade the servo signal and recording data, both of which are to be written later.